1. Field of the Invention
This invention relates to photoactive coatings and to methods of changing or obtaining the phase of a material, e.g., an anatase crystalline phase of titanium oxide from an amorphous phase of titanium oxide or from titanium metal and, more particularly, to methods of obtaining a photoactively hydrophilic and/or photocatalytic coating, and/or to articles made thereby.
2. Technical Considerations
For many substrates, e.g., glass substrates such as architectural windows, automotive transparencies, and aircraft windows, it is desirable that the surface of the substrate is substantially free of surface contaminants, such as common organic and inorganic surface contaminants, for as long a duration as possible. Traditionally, this has meant that these surfaces are cleaned frequently. This cleaning operation is typically performed by manually wiping the surface with or without the aid of chemical cleaning solutions. This approach can be labor, time, and/or cost intensive. Therefore, a need exists for methods to clean glass substrates that reduce the frequency and/or need for such manual cleaning operations.
It is known that certain semiconductor metal oxides provide a photoactive (hereinafter xe2x80x9cPAxe2x80x9d) coating. The terms xe2x80x9cphotoactivexe2x80x9d or xe2x80x9cphotoactivelyxe2x80x9d refer to the photogeneration of a hole-electron pair when illuminated by electromagnetic radiation of a particular frequency, typically ultraviolet (xe2x80x9cUVxe2x80x9d) light. Above a certain minimum thickness, these PA coatings are typically photocatalytic (hereinafter xe2x80x9cPCxe2x80x9d). By xe2x80x9cphotocatalyticxe2x80x9d is meant a coating which upon exposure to certain electromagnetic radiation, such as UV, interacts with organic contaminants on the coating surface to degrade or decompose the organic contaminants. With sufficient PC activity, these PC coatings are also self-cleaning. By xe2x80x9cself-cleaningxe2x80x9d is meant having sufficient PC activity to decompose organic contaminants fast enough that manual wiping to remove organic contaminants is not required. In addition, PC coatings are also typically hydrophilic. By xe2x80x9chydrophilicxe2x80x9d is meant water wetting with a contact angle with water of generally less than 20 degrees. The hydrophilicity of the PC coatings helps reduce fogging, i.e., the accumulation of water droplets on the coating, which may decrease visible light transmission and visibility through the coated substrate.
Titanium dioxide (TiO2) coatings are known to have hydrophilic and/or self-cleaning properties. However, not all phases of titanium dioxide are acceptable for providing self-cleaning and/or hydrophilic coatings. It is currently preferred to use the anatase crystalline phase rather than the amorphous phase or rutile crystalline phase of titanium dioxide to form PC coatings.
Sputter coating titanium dioxide, e.g., as a protective overcoat, has been used and is disclosed in U.S. Pat. No. 4,716,086. A limitation of conventionally sputter depositing titanium dioxide is that the anatase crystalline phase is not obtained. Another limitation is that sputter depositing a metal film is more efficient than depositing a metal oxide film. In the instance where a metal oxide film is desired, an efficient method is to sputter deposit a metal film on a substrate, and thereafter heat the deposited metal film in air. In the case of sputter deposited titanium metal film, the oxide film formed after heating is usually not the anatase phase but rather the rutile phase of titanium dioxide. Publications directed to the formation of titanium dioxide coatings on a glass substrate include U.S. Pat. Nos. 5,595,813 and 6,027,766, and xe2x80x9cPhotooxidative Self-cleaning Transparent Titanium Dioxide Films on Glassxe2x80x9d, Paz et al., J. Mater. Res., Vol. 10, No. 11, pp. 2842-48 (November 1995).
As can be appreciated, it would be advantageous to provide a method of making a sputter deposited hydrophilic and/or photocatalytic coating, e.g., by heating sputter deposited titanium metal films to convert the films to titanium dioxide films that are at least partly in the anatase phase.
This invention relates to a method of changing or obtaining the phase of a material and includes depositing a film that enhances the change or preferred deposition of a phase. In an embodiment of the invention, a titanium oxide film is deposited on a zirconium oxide film in the cubic or orthorhombic phase. In one embodiment of the invention, the deposited titanium oxide film is in the anatase phase. In another embodiment, a titanium metal film is deposited on a zirconium oxide film in the cubic or orthorhombic phase and the titanium metal film is heated in the presence of oxygen to provide a titanium oxide, e.g., titanium dioxide, film at least partly in the anatase phase.
In another embodiment of the invention, methods are provided for making a photoactive, e.g., photoactively hydrophilic and/or photocatalytic, coating. One method for making a photoactive coating includes depositing a first coating layer comprising zirconium oxide over at least a portion of a substrate and depositing a second coating layer comprising a photoactive material, such as titanium dioxide, over at least a portion of the first coating layer to provide a coated substrate. In one embodiment, the method includes heating at least one of the substrate, and/or the first coating layer, and/or the second coating layer to make the photoactive article.
The invention also relates to articles, e.g., windows for residential and commercial use, windows for land, air, sea, space and underwater vehicles, made using coated substrates of the invention. In one embodiment, the article includes a substrate, a zirconium oxide layer having a thickness of 10 xc3x85 to 200 xc3x85 deposited over at least a portion of the substrate, and a titanium oxide layer deposited over the zirconium oxide layer. In another embodiment, the article includes a first layer including a first material in a cubic or orthorhombic crystalline phase deposited over at least a portion of a substrate. A second layer having at least one photoactive material is deposited over, e.g., on the first layer.